1. Field of the Invention
The present invention relates to electronic devices equipped with electronic circuits that are driven by power supply means such as a battery, and a low-voltage detection method for detecting low voltage.
2. Description of Related Art
Portable electronic devices are well known. Many of these portable electronic devices are battery driven and driven by a primary or secondary battery. Some examples of these kinds of electronic devices are, portable game devices, portable audio devices and cellular telephones.
There are some battery driven electronic devices that have a mechanism for monitoring, displaying the amount of remaining charge of the battery. In these kinds battery such as electronic devices, there is a low-voltage detection circuit for monitoring the power-supply voltage supplied to the internal circuits.
FIG. 1 is a block diagram showing the configuration of a conventional low-voltage detection circuit. As shown in FIG. 1, the low-voltage detection circuit 101 is equipped with a BGR circuit 102 and a comparator 103.
The BGR circuit 102 is a constant-voltage output circuit called the "Band Gap Reference." The BGR circuit 102 outputs a predetermined absolute reference voltage Vref.
The comparator 103 compares the reference voltage Vref and the actual power-supply voltage Vlevel, and when the power-supply voltage Vlevel drops below the reference voltage Vref, it outputs a low-voltage detection signal Vdd_Low. The system uses this Vdd_Low signal as a trigger to perform necessary processing such as displaying "Battery End" or to save data for the application being executed.
However, there is error in the accuracy of the reference voltage output by the BGR circuit 102 and the threshold voltage of the comparator 103. Since electronic devices are designed for the worst-case condition which takes this error into consideration, "Battery End" is sometimes displayed for batteries that may still be used.
Moreover, the reference voltage Vref is generally determined to correspond to the lowest operable voltage for the internal circuits, however, the actual lowest operable voltage of the elements of the internal circuits, such as the semiconductors or the like, may vary depending on the manufacturing process or operating conditions. Therefore, depending on the variation of elements and operating conditions, "Battery End" was sometimes displayed for batteries that may still be used.
In other words, in conventional electronic devices, the effective battery capacity is not always completely used.